Spontaneous reaction predicting

Although thermodynamics can be used to predict the direction and extent of chemical change, it does not tell us how the reaction takes place or how fast. We have seen that some spontaneous reactions—such as the decomposition of benzene into carbon and hydrogen—do not seem to proceed at all, whereas other reactions—such as proton transfer reactions—reach equilibrium very rapidly. In this chapter, we examine the intimate details of how reactions proceed, what determines their rates, and how to control those rates. The study of the rates of chemical reactions is called chemical kinetics. When studying thermodynamics, we consider only the initial and final states of a chemical process (its origin and destination) and ignore what happens between them (the journey itself, with all its obstacles). In chemical kinetics, we are interested only in the journey—the changes that take place in the course of reactions. 

As described in the first part of this chapter, chemical thermodynamics can be used to predict whether a reaction will proceed spontaneously. However, thermodynamics does not provide any insight into how fast this reaction will proceed. This is an important consideration since time scales for spontaneous reactions can vary from nanoseconds to years. Chemical kinetics provides information on reaction rates that thermodynamics cannot. Used in concert, thermodynamics and kinetics can provide valuable insight into the chemical reactions involved in global biogeochemical cycles. 

AH can also indicate whether a reaction will be spontaneous. A negative (exothermic) value of AH is associated with a spontaneous reaction. However, in many reactions this is not the case. There is another factor to consider in predicting a reaction s spontaneity. We will cover this other factor a little later in this chapter. 

One of the goals of chemists is to be able to predict whether or not a reaction will be spontaneous. Some general guidelines for a spontaneous reaction have already been presented (negative A//and positive AS), but neither is a reliable predictor by itself. Temperature also plays a part. A thermodynamic factor that takes into account the entropy, enthalpy, and temperature of the reaction should be the best indicator of spontaneity. This factor is called the Gibbs free energy. 

A mixture of gaseous N2, H2, and NH3, each at a partial pressure of 1 atm, reacts spontaneously at 300 K to convert some of the N2 and H2 to NH3. We can predict the direction of spontaneous reaction from the relative values of the equilibrium constant K and the reaction quotient Q (Section 13.5). Since Kp = 4.4 X 105 at 300 K and Qp = 1 for partial pressures of 1 atm, the reaction will proceed in the forward direction because Qp is less than Kp. Under these conditions, the reverse reaction is nonspontaneous. At 700 K, however, Kp = 8.8 X 10 5, and the reverse reaction is spontaneous because Qp is greater than Kp. 

Thermodynamics deals with the interconversion of heat and other forms of energy and allows us to predict the direction and extent of chemical reactions and other spontaneous processes. A spontaneous process proceeds on its own without any external influence. All spontaneous reactions move toward equilibrium. 

The first law of thermodynamics cannot be used to predict whether a reaction can occur spontaneously, as some spontaneous reactions have a positive AE. Therefore a function different from AE is required. One such function is entropy (S), which is a measure of the degree of randomness or disorder of a system. The entropy of a system increases (AS is positive) when the system becomes more disordered. The second law of thermodynamics states that a process can occur spontaneously only if the sum of the entropies of the system and its surroundings increases (or that the universe tends towards maximum disorder), that is ... 

There is also a separate expert system for the combination with MultiCASE, which predicts the possible metabolites, formed of a compound. This system is known as META, which was developed to identify molecular sites susceptible to metabolic transformation. The metabolism dictionary associated with META is able to recognize 663 enzyme-catalyzed reaction rules, which have been categorized into 29 enzyme-reaction classes and 286 spontaneous reactions (Klopman and Rosenkranz 1994). 

Spontaneous reaction of hydrogen peroxide/sulfuric acid/water/organic mixtures. These reactions can accelerate rapidly and terminate violently, and can be outside the predicted explosive area. 

We can find out whether a proposed reaction is possible by determining whether it is a spontaneous thermodynamic process. In this context, spontaneous has a precise technical meaning (see later for clarification) that should not be confused with its conversational meaning, such as describing the spontaneous behavior of people in social situations. Thermodynamics can tell us whether a proposed reaction is possible under particular conditions even before we attempt the reaction. If the reaction is spontaneous, thermodynamics can also predict the ratio of products and reactants at equilibrium. But, we cannot use thermodynamics to predict the rate of a spontaneous reaction or how long it will take to reach equilibrium. These questions are the subject of chemical kinetics. To obtain a large amount of product from a spontaneous reaction in a short time, we need a reaction that is spontaneous and fast. 

These three examples illustrate that the actual outcome of a spontaneous chemical reaction depends on the reaction rate. The possibility of reaction between hydrogen and oxygen was there all along, but the rate was too slow to be observed until the powdered metal or the electrical spark accelerated the reaction. The possibility of reaction between metallic copper and oxygen was there all along, and the rate was large enough to be observed, if not dramatically fast. Thermodynamics determines whether a reaction is possible, whereas chemical kinetics determines whether it is practical. At the end of this chapter, you will be able to predict whether a chemical reaction is spontaneous, and by the end of the next chapter, you will be able to predict its equilibrium state. But, you must wait until Chapter 18 to see whether a spontaneous reaction can be carried out at a useful rate. 

The specific examples in Section 14.5 demonstrate that when 1C > 1 the reaction has progressed far toward products, and when K 1 the reaction has remained near reactants. The empirical discussion in Section 14.6 shows how the reaction quotient Q and the principle of Te Chatelier can predict the direction of spontaneous reaction and the response of an equilibrium state to an external perturbation. Here, we use the thermodynamic description of K from Section 14.3 to provide the thermodynamic basis for these results obtained empirically in Sections 14.5 and 14.6. We identify those thermodynamic factors that determine the magnitude of K. We also provide a thermodynamic criterion for predicting the direction in which a reaction proceeds from a given initial condition. 

An example of how a free energy calculation can be used to predict whether a spontaneous reaction between a solution and solid phase is possible is the oceanographically important process of calcite (CaC03(s)) mineral dissolution. The question is whether this reaction... 

Suppose we ask the question At standard conditions, will Cu + ions oxidize metallic Zn to Zn + ions, or will Zn + ions oxidize metallic copper to Cu + One of the two possible reactions is spontaneous, and the reverse reaction is nonspontaneous. We must determine which one is spontaneous. We already know the answer to this question from experimental results (see Section 21-9), but let us demonstrate the procedure for predicting the spontaneous reaction. 

Reaction (i) is therefore more favourable because it has a higher (more positive) standard potential. Both reactions agree with the predictions made in Exercise 14.5 because lead and tin are going to their most stable oxidation States. Keep in mind that a positive reaction potential indicates a spontaneous reaction. 

We saw earlier that one can predict the thermodynamic open circuit (zero current) potential of an electrochemical cell by combining two half-cell reactions, one for the anode and the second for the cathode. The half-cell reaction with the lower (i.e., more negative) equilibrium potential will proceed spontaneously in the anodic direction, where the electrode acts as an electron sink for the anodic de-electronation (oxidation) reaction and the higher equilibrium potential reaction will occur spontaneously at the cathode, where the electrode acts as an electron source for the electronation (reduction) reaction. A cell with spontaneous reactions at the anode and cathode is called a self-... 

In order to predict the sign of AG, according to Equation (18.7) we need to know both A7f and AA. A negative A// (an exothermic reaction) and a positive Ag (a reaction that results in an increase in disorder of the system) tend to make AG negative, although temperature may influence the direction of a spontaneous reaction. The four possible outcomes of this relationship are ... 

There is no coimection between the spontaneity of a reaction predicted by AG and the rate at which the... 

Given below is the standard electrode potential (E ) of the following redox reaction. Predict the most feasible event, if the reaction occurred spontaneously. 

There is no coimection between the spontaneity of a reaction predicted by AG and the rate at which the reaction occurs. A negative free energy change tells us that a reaction has the potential to happen, but gives no indication of the rate. 

We are looking for a chemical energy term that will always decrease in such spontaneous reactions and will enable us to systematize and predict what way reactions will proceed under given conditions. This may seem like a simple problem, but it is not. 

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